1. Field of the Invention
This invention relates to a raster scan type cathode ray tube (CRT) display device for displaying characters and graphs, and more particularly relates to a graphic generator for shading graphs.
2. Description of the Prior Art
Shading capabilities have been developed for the graphic displays provided by raster scan type CRT display devices which enhances user interface with the computer.
As shown in FIG. 1, in the prior art, the shading operations are performed to shade regions defined by one straight line L.sub.1 connecting two points P.sub.1 and P.sub.2 and another straight line L.sub.2 parrallel to the Y axis (or X axis). As is currently being done in the field of the raster scan type graphic display devices, the displaying regions are divided into small equal sections as shown in FIG. 2 and it must be determined whether each section is to be lightened or not. In this case, the graphic display device simulates all graphs, including straight lines, by displaying (lightening) a group of the section. The graphic display device stores the information to lighten the sections in the memory regions corresponding to the display regions. The graphic display device provides the axes information to lighten the sections according to the information in the memory of the controller to generate the graphic display. Therefore, it is very important how the graphic display device provides the axes information for the shading.
In the prior art, there are two methods used to obtain the axes information for the shading. One method calculates all the axes of the sections to be lightened by microprocessors and the other separately processes two regions by hardware modules to shade a right-angle triangle. In these methods, at least the axes of an intersection P.sub.3 must be calculated and it therefore takes a long time to process. If the line L.sub.1 does not cross the line L.sub.2, it is necessary to operate using other hardware and programs.